In conventional oil and gas filed well drilling operations, it requires putting casings downhole to realize stratum blocking in order to ensure safety and favorable arrival in the target layer. In a wellbore casing structure sequence, as the number of casing layers in the well increases, the casing size and the borehole size are progressively reduced, and the borehole taper is progressively increased, so that the target depth cannot be reached in deep, ultra-deep and complex wells or the subsequent operations will be affected by too small sizes of boreholes.
Therefore, it requires to improve wellbore structures and drilling and completion techniques to ensure a constant borehole size and completion inner diameter and thus to drill deeper wells.